How to have a race when your boat doesn't move!
Concept 2's performance monitor (PM) uses signals from a sensor to detect the motion of the flywheel, much the
same as a cycle computer uses a sensor mounted on the front forks of your bike to detect the motion of your wheel. Both
have a built-in "clock" to measure time intervals required for the calculations.

However, unlike the cycle computer, the PM can't use distance or speed to monitor performance simply because
a rowing machine doesn't actually travel anywhere ... there's no speed or distance to measure! Instead, the
PM calculates the power applied to the handle to gauge how hard you're rowing.

How is it able to do this?

Power applied to the handle: a measure of rowing intensity
Physics tells us that work is done when a force applied to an object causes it to move. For example: a person pulling
on the handle of a rowing machine does work. Now, without getting too technical, it turns out that the work done in
pulling the handle during the rowing cycle can be calculated with knowledge of:

The motion of the flywheel during the drive

Some physical quantities associated with the rotating flywheel-fan assembly (including something called the
'moment of inertia' - which depends on the mass and how it's distributed throughout the assembly)

The motion of the flywheel during the recovery.

The rate at which the flywheel slows down during the recovery phase of the stroke allows the drag force (air
resistance on the fan, friction in the bearings etc.) to be calculated and taken into account. Concept 2 tightly control
the consistency of the moment of inertia of the rotational assembly from one rowing machine to another
and its value is built into the calculations done by the PM. This allows the work done in each rowing stroke
to be accurately computed.

By dividing the amount of work done at the handle by the time taken to complete a stroke the PM calculates the average
power over the stroke. The power, in Watts, is one of three different measures of rowing intensity that the PM
can display.

'Row, row, row your notional boat'
In order to introduce a more familiar concept to the measurement of an indoor rower's performance, the PM uses a
formula which relates the stroke power to the speed that an on-the-water boat may typically travel if the oarsman were
to supply the same power to propel his or her boat. For the mathematically minded, the relationship between power and
'boat speed' used by Concept 2 is:

Power = 2.8 x Speed 3 (Power in Watts,
speed in metres/second.)

For example, power of 350W at the handle corresponds to a speed of 5m/s because 2.8 x 53 = 350.

Although this 'speed' is an abstract quantity, its mathematical relationship to the actual power at the handle makes it
a perfectly valid quantity for comparing rowing intensities.

With a means of deriving a 'speed' for each stroke, it becomes a simple matter to calculate the 'distance'
travelled by our notional boat over a given period of time. The accuracy with which Concept 2 rowing machines measure
power and the formula linking power to speed are what make competitive indoor rowing over fixed 'distances' a practical
reality.

Two more expressions of rowing intensity: pace and energy expenditure
During a race, knowledge of your pace (i.e. the time taken for your notional boat to travel a fixed distance),
rather than your speed, makes it easier to predict your finish time. So in preference to displaying speed, the PM can
display the pace in minutes and seconds per 500m - for each stroke, averaged over a split or averaged over the entire
row.

Using our earlier example: at a 'speed' of 5m/s you would take 100s to 'travel' 500m. So a rowing intensity
of 350 Watts is equivalent to a pace of 1:40/500m.

Finally, an attempt has been made by the PM designers to relate the power exerted at the handle to the rate
of total energy expended, in Calories per hour, by the user while rowing. This is based on two arbitrary
assumptions:

The rower expends energy at the rate of 300 Cal/hr performing all functions other than applying power
to the handle. (This includes basal metabolism plus the effort required to perform the rowing movement up and down
the monorail.)

Of the additional energy expended, only 25% manifests itself as mechanical power applied to the handle.

The formula for energy expenditure is only an approximation based on a rower weighing 175lb and may not be very accurate
for any given individual. For lighter or heavier rowers, the factor can be expected to differ from 300 Cal/hr, roughly
in proportion to the rower’s weight.